The present invention relates to semiconductor memory devices, and more particularly, to NAND flash memory devices.
Generally, a semiconductor memory device is a memory device for storing data and retrieving target data by reading stored data. Semiconductor memory devices may be classified as random access memory (RAM) and read only memory (ROM). RAM is volatile memory that loses stored data when power is interrupted. ROM is nonvolatile memory that holds stored data even when power is interrupted. RAM includes dynamic RAM and a static RAM. ROM includes programmable ROM, erasable ROM, electrically programmable ROM (EPROM), and flash memory. Flash memory devices may be classified as NOR type and NAND type.
FIG. 1 is a circuit diagram showing a cell string structure of a conventional NAND flash memory device. The cell string structure 1 of a NAND flash memory device shown in FIG. 1 is described in U.S. Patent Publication No. 2004/0113199.
Referring to FIG. 1, one cell string includes 16 memory cells MC0-MC15 connected in series. Word lines WL0-WL15 are connected to respective gates of the memory cells MC0-MC15. Respective selection transistors ST1 and ST2 connect the cell string to a bit line BL and a common source line CSL. The selection transistor ST1 connected to the bit line BL is called a string selection transistor, and the selection transistor ST2 connected to the common source line CSL is called a ground selection transistor. The gate of the string selection transistor ST1 is connected to a string selection line SSL and the gate of the ground selection transistor ST2 is connected to a ground selection line GSL.
A dummy memory cell DC0 is connected between the string selection transistor ST1 and the memory cell MC0, and another dummy memory cell DC1 is connected between the ground selection transistor ST2 and the memory cell MC15. Gates of the dummy memory cells DC0 and DC 1 are connected to respective dummy word lines DWL0 and DWL1. The dummy memory cells DC0 and DC1 have substantially the same structure as the memory cells MC0-MC15. However, the dummy memory cells DC0 and DC1 do not perform program and a read operations, that is, the dummy memory cells DC0 and DC1 are not used as a data storage elements.
FIG. 2 is a table showing bias voltage conditions for read, erase and program operations for the cell string structure shown in FIG. 1. Biasing for the read operation is as follows. The bit line BL is pre-charged by applying 0.5V thereto. A power supply voltage Vcc is applied to the string selection line SSL and the ground selection line GSL. 0V is applied to the common source line CSL. A read voltage is applied to a selected word line and a predetermined voltage is applied to non-selected word lines and the dummy word line to turn on the memory cells. Then, 0V is applied to a P type substrate P_Well of the device.
Biasing for the erase operation is as follows. The bit line BL, the string selection line SSL, the ground selection line GSL and the common source line CSL are placed in a floating state. An erase voltage of 18V is applied to the P type substrate P_Well. Then, 0V is applied to all of the word lines WL and dummy word lines DWL.
Biasing for the program operation is as follows. 0V is applied to a bit line of a memory cell to be programmed. A power supply voltage Vcc is applied to a bit line of cells that are not to be programmed. The power supply voltage Vcc is applied to the string selection line SSL, and 0V is applied to the ground source line GSL and the common source line CSL. The program voltage Vpgm, e.g., 18V, is applied to a selected word line, and a pass voltage Vpass, e.g., 8V, is applied to non-selected word lines. A voltage identical to the pass voltage Vpass is applied to the dummy word lines DWL.
Because 18V is applied to the gate of a cell being programmed and the channel voltage is 0V, a strong electric field is generated between the gate and the channel of the cell being programmed. Electrons in the channel of the programmed cell are injected into the floating gate due to Fowler Nordheim (F-N) tunneling. An inhibited cell has a gate voltage of 18V and a channel voltage of (power supply voltage Vcc—threshold voltage Vth), where Vth is a threshold voltage of the selection transistor ST1. However, the channel voltage of the inhibited cell increases to about 8V due to a capacitive boosting effect formed between the gate and the channel, that is, an insufficient electric field is generated between the gate and the channel of program inhibit cell to cause F-N tunneling. Therefore, the inhibited cell is not programmed.
FIG. 3 is a cross-sectional view of the cell string structure shown in FIG. 1 tinder a bias condition for a program inhibit cell. Referring to FIG. 3, 0V is applied to a common source line CSL and a ground selection line GSL, and 18V is supplied to a selected word line WL15. 8V is applied to a non-selected word line WL14 and a dummy word line DWL1. As a result, channel voltages of the memory cells and the dummy memory cell increase to about 8V.
In order to prevent the program inhibit cell from being programmed, it is desirable that the channel voltage increase due to the capacitive boosting effect be substantially maintained. Therefore, 0V is supplied to the ground selection line GSL while programming in order to place the ground selection transistor ST2 in a cut-off state, as shown in FIG. 3. As a result, the channel voltage increase arising from the capacitive boosting effect may be maintained by preventing leakage through the ground selection transistor ST2 while programming.
Although not shown in FIG. 3, such operations may be applied to the string selection transistor ST1. While programming, a power supply voltage Vcc is applied to a bit line BL and a string selection line SSL, and 18V is applied to a selected word line WL0. 8V is applied to a non-selected word line WL1 and a dummy word line DWL0. In this case, channel voltages of the memory cells and the dummy memory cell increase to 8V by the capacitive boosting effect. In order to prevent a program inhibited cell from being programmed, the string selection transistor may be placed in a cut-off state by applying a power supply voltage Vcc to the string selection line SSL while programming. Therefore, the channel voltage increase due to the capacitive boosting effect may be maintained by preventing leakage through the string selection transistor ST1 while performing the program operation.
However, the increased channel voltage of the program inhibit cell may leak due to various factors shown in FIG. 3. When about an 18V program voltage is applied to a selected word line WL15 and about an 8V pass voltage Vpass is applied to a dummy word line DWL1, the drain of the ground selection transistor ST2 has the increased channel voltage of about 8V. If the ground selection transistor ST2 has an insufficient channel length Ls, a leakage current IPNTR may be generated due to punch-through. As a result, the channel voltage may decrease. If the channel voltage decreases due to leakage current, the program inhibit cell may become programmed due to F-N tunneling. In order to prevent the program inhibit cell from being programmed, it is desirable to provide sufficient channel length Ls for the ground selection transistor ST2. However, a long channel may make it difficult to scale down the size of the cell string.
While performing a program operation, the gate voltage of the ground selection transistor ST2 is 0V and the drain voltage is about 8V, which is a relatively high voltage. If a high voltage of about 8V is applied between the drain and the gate of the ground selection transistor ST2, a leakage current IGIDL may flow from a drain region to a substrate region due to gate induced drain leakage (GIDL). The leakage current IGDL generated by the GDL may decrease the channel voltage. As a result, the program inhibit cell may be programmed.
Such a problem of programming a program inhibit cell may occur at a memory cell MC0 adjacent to the string selection transistor ST1. While performing the program operation, the gate voltage of string selection transistor ST1 is substantially equal to the power supply voltage Vcc and the drain voltage is about 8V. As a result, a high voltage of about 6 to 8V is applied between the drain and the gate of the string selection transistor ST1. Therefore, leakage current IGIDL may be generated at the string selection transistor ST1.
If about 8V is supplied to the dummy memory cell DC1 and the dummy word line DWL1 and 0V is supplied to the ground selection line GSL, a lateral electric field is formed between the channel of dummy memory cell DC1 and the channel of the ground selection transistor ST2. The lateral electric field generates an electron-hole pair (EHP) between the dummy memory cell DC1 and the ground selection transistor ST2. The electron of the EHP is accelerated to the channel of dummy memory cell DC1. The accelerated electron crashes into a silicon crystal Si, and another EHP is generated by the scattering. Such continuous scattering generates hot electrons, which may be injected to the floating gate of dummy memory cell DC1 by the strong vertical electric field. In this case, the threshold voltage of dummy memory cell DC1 may increase and the program inhibit cell may be programmed.
FIG. 4 is a cross-sectional view of the cell string structure shown in FIG. 1. FIG. 4 shows a bias voltage condition for the erase operation. While performing the erase operation, a dummy memory cell DC1 is erased with the memory cells MC14, MC15. However, the dummy memory cell DC1 is repeatedly erased, while the memory cells MC14, MC15 are erased and programmed. If erase operations are repeatedly performed on the dummy memory cell, a significant amount of positive charge may accumulate at the floating gate of the dummy memory cell. This may influence a cell current while performing a read operation, a program verification operation and an erase operation and, as a result, programming and erasing characteristics may be degraded.